moving struct bpf_if to bpf.c

[Mark Johnston]

Hello,

At the moment, bpf.h defines struct bpf_if differently depending on
whether BPF_INTERNAL is #defined. This causes problems with CTF, as it
results in a sort of bifurcation within the type graph: CTF sees two
different struct bpf_ifs, and so every struct/union containing a struct
bpf_if is duplicated, and so on. CTF currently imposes a limit of 2^15
distinct types within a container, and several people have been running
into this limit. The type duplication exacerbates this problem.

The change here fixes the issue by moving the definition of
struct bpf_if to bpf.c, and making its externally-used fields
available via struct bpf_if_ext: https://reviews.freebsd.org/D2319

In particular, the ext fields are at the same offsets within struct
bpf_if as before, so this change should have no functional impact.
Moreover, it reduces the number of types from 20879 to 15725 with my
(stripped-down) kernel config on amd64. Would anyone be willing to
review the proposed change? I've also placed the raw diff here:
https://people.freebsd.org/~markj/patches/bpf_entrails.diff

Thanks,
-Mark